A power transmission path between an engine and drive wheels is disposed with a plurality of pairs of gears meshed with each other and drive power of the engine is transmitted via these meshed gears to the drive wheels. Such a meshed gear is known to generate booming noise and rattling noise during power transmission. The booming noise is vehicle interior noise occurring because vibrations are caused by torque variation of the engine and transmitted via a transmission (gearbox) through a drive shaft and a suspension to the inside of a vehicle. The rattling noise is vehicle interior noise occurring because when vibrations are caused by torque variation of the engine and transmitted to the transmission, collision of gears with each other so-called rattling (tooth hitting) occurs in meshed gears in the transmission and the rattling causes a case surface of the transmission to vibrate and generate noise, which is transmitted to the inside of a vehicle. With regard to reduction of these booming and rattling noises, it is known that if gear inertia (moment of inertia) of gears disposed in the power transmission path is increased, the booming noise and the rattling noise can be improved since a level of response of gears to the torque variation of the engine is reduced.
It is also known that the meshed gear generates gear noise during power transmission. Meshing of gears (a driving gear and a driven gear) always has a rotation angle error relative to ideal rotational motion. The rotation angle error corresponds to an advance or a delay of the driven gear relative to the driving gear and is also referred to as a mesh transmission error. This mesh transmission error is generated due to a manufacturing error of a gear, an assembly error, and elastic deformation of a gear, a support shaft, a case, etc., and the mesh transmission error cannot be set to zero. The presence of the mesh transmission error generates a meshing point coercive force acting as a vibration source of the gear noise, resulting in the gear noise.
A mechanism of generation of the gear noise will be described with reference to FIG. 6. FIG. 6 is a schematic of a meshed state between a driving gear 150 and a driven gear 160. In FIG. 6, when X1 and X2 denote respective displacements (rotational displacements) of the driving gear 150 and the driven gear 160 at a meshing point, a mesh transmission error TE is expressed by the following Equation (1). When φ1 and φ2 denote respective compliances of the driving gear 150 and the driven gear 160 at the meshing point, the displacement X1 of the driving gear 150 and the displacement X2 of the driven gear 160 are expressed by the following Equations (2) and (3), respectively. F1 and F2 are coercive forces (meshing point coercive forces) at the meshing point of the driving gear 150 and the driven gear 160 and are in the relationship of F1=−F2 due to the action-reaction relationship. When X is a response displacement generated by applying a force F to a structure, the compliance φ is a physical amount acquired by dividing the displacement X by the force F and represented with a frequency axis. For example, when the compliance φ is smaller, the displacement X becomes smaller relative to the force F and when the compliance φ is larger, the displacement X becomes larger relative to the force F.TE=X1−X2  (1)X1=φ1×F1  (2)X2=φ2×F2  (3)
From Equations (1) to (3) and the relationship of F1=−F2, the following equation (4) is established. In Equation (4), 1/(φ+φ2) is defined as meshing point dynamic rigidity. It is known from Equation (4) that the meshing point coercive force F1 becomes smaller when the compliances φ1 and φ2 are made larger. Therefore, if the compliances φ1 and φ2 are made larger, the meshing point coercive force F1 becomes smaller and the gear noise is reduced.F1=TE/(φ1+φ2)  (4)
To reduce the meshing point coercive force F1, Patent Document 1 discloses a technique of reducing the meshing point dynamic rigidity by attaching an annular member (in a plate shape) on a side surface of a gear of a flange so that the annular member acts as an additional vibration system.